1. Technical Field
The present invention relates to radial lip seals and more particularly relates to seals which seal fluids between two relatively rotatable machine elements.
2. Brief Description of the Background Art
Radial lip oil seals are used in numerous rotating machine applications including wheels hubs, shaft journals and antifriction bearings. A particular form of a radial lip oil seal known as a unitized oil seal is preferred in certain applications because it protects the seal lips and minimizes the need for finishing the surfaces sealed by the unitized seal. A typical unitized radial lip seal includes relatively rotatable inner and outer metal elements or casings upon which elastomeric seal lips may be formed.
Unitized seals are often installed as a unit within a bore in a wheel hub. The wheel hub is then mounted over an axle around which the seal lip forms an annular sealing barrier. This mounting assembly generally involves some relative axial shifting of the inner and outer unitized seal elements. As the seal elements are axially shifted during installation, a significant axial load may be applied to the seal and may result in metal-to-metal, metal-to-rubber, and/or rubber-to-rubber contact between the inner and outer elements.
The large axial loads generated between the inner and outer seal elements during installation can bring about the formation of metal chips as the unitized seal is initially rotated and "broken-in." These metal chips can migrate beneath the seal lips causing abrasions and cuts in the lips and eventual seal failure. Such metal-to-metal contact can thereby reduce seal life and cause an increase in torque required to rotate the seal as the inner and outer metal elements rub against one another with significant friction.
In order to prevent metal-to-metal contact, prior unitized seals have used axially-extending elastomeric bumpers or nibs for axially spacing the inner seal element from the outer seal element. However, upon mounting the unitized seal within a housing bore and over a shaft, the axially-directed mounting forces axially compress the bumpers and nibs between the inner and outer sealing elements. During the initial break-in period of the unitized seal, the nibs or bumpers on one seal element must be worn away to eventually provide clearance or minimal contact between the bumpers and a locating surface on the other seal element.
During the break-in period, the rubber nibs or bumpers generate significant resistance to rotation between the inner and outer sealing elements. This resistance must be overcome by increasing the torque applied between the sealing elements. Clearly, this increased resistance is undesirable from an efficiency viewpoint as energy is required to overcome the friction generated by the abrasion of the bumpers and nibs. Moreover, the heat generated by the friction can adversely affect the seal lip materials and the abraded rubber can cause at least temporary seal leakage as the abraded rubber particles work their way under the seal lips.
Because the nibs are initially compressed during installation, they subsequently expand axially during break-in thereby prolonging the time during which start-up torque must be increased to generate relative rotation between the shaft and bore being sealed. That is, as the nibs are worn away, they still maintain contact with the other seal element as they axially expand to relative their compression. This prolongs the break-in period.
A typical example of a continuous unitized oil seal design is shown in U.S. Pat. No. 3,510,138 to Bowen which discloses the concept of providing a hard rubber surface on a unitized oil seal to minimize squealing of new seals when a seal case binds against an elastomeric portion of an oil seal. The hard rubber washer is bonded to the elastomeric body of the seal. If the seal of Bowen is assembled with excessive axial loading, the hard rubber washer will compress the elastomeric portion and result in increased torque being required to rotate the seal. Since the hard rubber washer is bonded to an elastomeric member, assembly of the seal is fairly complex and requires a difficult molding step.
Accordingly, a need exists for a unitized seal which reduces or eliminates the formation of abraded metal and/or rubber particles during break-in. A further need exists for a unitized oil seal which minimizes its break-in period and which significantly reduces initial or "start-up" torque requirements for initially rotating the newly installed seal. Yet another need exists for a unitized oil seal which, after break-in, generates minimal resistance to rotation.